Cable construction containing optical fibers and reinforcement means

ABSTRACT

A cable construction comprising at least one package (3) of fibre ribbons, which package consists of at least one fibre ribbon (4) comprising several parallel optical fibres (5) and a covering (6) joining them together, and at least one reinforcing member (7a) extending in the longitudinal direction of the cable. The package (3) of fibre ribbons is arranged in a cavity (2) with an elongated cross-section in the vertical direction such that the natural bending plane (A) of the fibre ribbon package (3) is substantially perpendicular with respect to the longitudinal plane (B) of the cavity. The reinforcing member (7a) is arranged in the cable construction in such a way that the bending plane (D) of the whole cable defined thereby also is substantially perpendicular with respect to the longitudinal plane (B) of the cavity.

FIELD OF THE INVENTION

The invention relates to a cable, comprising (i) at least one package offibre ribbons, which package consists of at least one fibre ribboncomprising several parallel optical fibres and a covering joining themtogether, and (ii) at least one reinforcing member extending in thelongitudinal direction of the cable.

BACKGROUND AND PRIOR ART

It is known to use in a cable optical fibre ribbons, comprisingtypically 4 to 12 parallel optical fibres and a covering manufactured ofplastic material joining them together.

In a known cable construction, fibre ribbons are normally packagedtightly inside a plastic tube and the plastic tube is surrounded by aprotective structure comprising reinforcing members extending in thelongitudinal direction of the cable, which members are placed at regularintervals in the peripheral direction of the cable.

Another known cable construction is a so-called V-groove cable, in whichfibre ribbons are placed in spirally or oscillatingly winding groovespositioned in a common centre element.

A drawback of these known cable constructions is that, due to thecomplicated structures thereof, the production costs are high. In theabovementioned constructions, it is, furthermore, not possible toprovide in a simple manner a proper play for the fibres against changingdimensions caused by mechanical forces directed to protective sheath orby thermal effects.

SUMMARY OF THE INVENTION

The object of the present invention is thus to avoid the drawbacksdescribed above. This is achieved by means of a cable constructionaccording to the invention, which is characterized by a combination inwhich (i) at least one package of fibre ribbons is arranged in a cavitywith an elongated cross-section in such a way that the natural bendingplane of the fibre ribbon package is substantially perpendicular withrespect to the longitudinal plane of the cavity, and (ii) at least onereinforcing member is arranged in the cable construction in such a waythat the bending plane of the whole cable defined thereby also issubstantially perpendicular with respect to the longitudinal plane ofthe cavity.

The basic idea according to the invention is to arrange the fibreribbons and the reinforcing members with respect to the elongated cavityformed in the cable in such a way that the fibre ribbons are allowed tomove inside the cavity, while extending, compressing or bending thecable, in such a manner that they bend with respect to their naturalbending plane.

Thanks to the construction of the invention, the fibres can in an easyway be provided with a large play. Due to its simple structure, thecable of the invention also keeps the production costs low, and thereby,a cable network can be constructed more economically than before. Thesolution according to the invention improves the profitabilityespecially at the lowest level of an optical cable network, i.e. withinthe area of a local network, where the number of fibres is highest andwhere the need to achieve cheaper solutions than the present ones isgreatest.

In the construction of the invention, the cavity, the fibre ribbons andthe reinforcing members are mutually arranged in such a way that amanufacture by means of a stationary (not turning) tool is made possibleand the fibres can be provided with the extra length they need withrespect to the protective sheath simply by allowing them to bend withrespect to their natural bending plane. Moreover, the cable geometry ofthe invention allows a minimization of the cross-section of the cable,and consequently, an increase in the packing density of the cable.

According to one preferred embodiment of the invention, the reinforcingmembers consist of a pair of metallic conductors, whereby they can beused also for data transmission or e.g. supply of power.

According to a second preferred embodiment of the invention, thereinforcing members confine the cavity space within themselves andprotect it. Thanks to such a construction, it is easy to branch thecable without damaging the fibres.

According to a third preferred embodiment of the invention, the cablecore itself or a part of it constitutes a reinforcing member. Noseparate reinforcing members integrated inside the construction are thenneeded, but the cable core or a part of it determines the properties ofthe cable alone.

The cable geometry according to the invention makes it also possible toprovide the protective structure with sliding/wear surfaces working aswear surfaces and reducing friction when the cable is drawn.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the following descriptionwith reference to the examples according to the enclosed drawings, inwhich

FIG. 1 shows a cross-section of a cable according to a first embodimentof the invention,

FIG. 2 shows a cross-section of a cable according to a second embodimentof the invention,

FIG. 3 shows a cross-section of a cable according to a third embodimentof the invention, and

FIGS. 4a to 4c show cross-sections of different alternatives of a cableaccording to a fourth embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a cross-section of a cable construction according to afirst embodiment of the invention, in which construction in a protectivesheath 1 made of cable plastic, e.g. polymer, is formed a cavity 2extending in the longitudinal direction of the cable, in which cavity isarranged a package 3 of fibre ribbons, in this case consisting of threefibre ribbons 4 one upon another and optionally tied together. Eachfibre ribbon comprises four optical fibres 5 placed in parallel witheach other and a covering 6 joining them together, which covering istypically made of acrylate plastic. Inside the protective sheath 1 arealso arranged two threadlike reinforcing members 7a extending in thelongitudinal direction of the cable.

The cavity 2 is situated centrally with respect to the longitudinalcentre axis of the cable and it has a cross-section with an elongatedform along a diametral axis in the vertical direction in FIG. 1. Thefibre ribbon package 3 is arranged inside the cavity in such a mannerthat its natural bending plane, indicated by the reference character A,is perpendicular with respect to the longitudinal plane B. Thelongitudinal plane B is perpendicular to the cross-sectional plane ofthe cable and parallel with the longitudinal axis of the cavity (asymmetry axis parallel with a long side 2b of the cavity). Inside thecavity 2, the fibre ribbons extend according to the invention along apath similar to that of a sine curve, oscillating with respect to theirnatural bending plane. On account of the oscillating path, the fibreribbons have a specific excess length and thus also an allowance forstrain with respect to the protective sheath. The natural bending planeof a fibre ribbon package means in this connection a plane on both sidesof which the fibre ribbons extend and which is perpendicular to thecross-sectional plane of the cable and at the same time parallel withthe transverse direction defined by one fibre ribbon 3.

The width W of the cavity in the horizontal direction of is such thatthe fibre ribbon package is not able to move substantially in saiddirection, while the length L of the cavity in the vertical direction issuch that the fibre package has play in this direction. In practice, thelength L of the cavity is typically three- to fivefold compared with itswidth W.

The reinforcing members 7a are arranged on opposite sides of the cavityin such a way that the bending plane D of the whole cable defined bythem is also perpendicular with respect to the longitudinal plane B ofthe cavity. In this case, with the fibre ribbon package locatedcentrally in the cavity 2, the bending plane D coincides with thebending plane A of the fibre ribbon package. The bending plane of thecable means in this example a level which is perpendicular to thecross-sectional plane of the cable and passes through the centre axes ofthe reinforcing members 7a.

Because the reinforcing members 7a are arranged on the main axis A ofthe cavity 2, at which the cavity is narrowest (at the transverse axis),the cross-section of the cable can be made as small as possible, andconsequently, the packing density high.

Moreover, because the reinforcing members 7a are arranged symmetricallyon opposite sides of the cavity, they can be made as a pair ofconductors, which can be used for data transmission, e.g. fortransmission of service signals, or for supply of power. In this case,it is preferable to use as reinforcing members copper or aluminum coatedsteel wires, which make both reinforcing and data transmission functionspossible. In case the reinforcing members 7a are not used for datatransmission, they can be e.g. mere steel wires or composite plasticbars (FRP=Fiber Reinforced Plastics).

Thanks to the location of the reinforcing members, the cable can also bebranched easily, because the sheath 1 of an undamaged cable can bestripped in the direction of the reinforcing members, whereby the fibreribbons are easily accessible without breaking them. An especiallypreferably embodiment in this respect is the construction shown in FIG.2, which will be described later on.

From the cable geometry according to the invention it follows also thatat bends of a channel installation only those areas of the outer surfaceof the protective sheath 1 of the cable touch the wall of the channelwhich are closest to the end parts 2a of the cavity 2. Due to this, itis preferably to provide the protective sheath with sliding/wearsurfaces functioning as wear surfaces and reducing friction when thecable is drawn. Such sliding surfaces, indicated by the referencenumeral 8, can be realized e.g. by means of a reinforcement made on thesurface of the protective sheath, which reinforcement is provided withgrooves 8a in the longitudinal direction of the cable.

FIG. 2 shows a cross-section of a cable according to a second embodimentof the invention. In this case, reinforcing members 7b are semicircularin cross-section and they are in the direction of the longitudinal axisof the cavity at least as long as the cavity, preferably a little longerthan the cavity, however, whereby they thus confine the cavity space andprotect it. Then the fibre ribbons are easily accessible by strippingthe protective sheath 1 along a plane defined by the reinforcing members7b, i.e. along the plane C indicated in FIG. 2 by a broken line.

FIG. 3 shows a third embodiment of the cable construction of theinvention, in which the reinforcing member consists of one metallic band7c, the cross-sectional form of which is bent in such a way that fourmutually parallel walls form three parallel substantially U-shapedspaces. At the manufacturing stage of the cable, tape 9 has been woundaround the reinforcing member 7c, whereby the U-shaped spaces are closedand three parallel cavities 2c are formed for fibre packages 3. In thiscase, the cross-sectional form of the metallic band 7c defines thelocation of the bending plane D of the cable.

The reinforcing member can also be realized in the manner shown in FIGS.4a to 4c, by forming the protective sheath 1 in two parts in such a waythat a cable core 7d (or at least a substantial part of it) ismanufactured of a rigid plastic or composite material and thesurrounding protective sheath 1 of a softer and cheaper plasticmaterial. Consequently, the cable core 7d (or the substantial part ofit) acts as a reinforcing member defining the bending plane D of thewhole cable.

In the case of FIG. 4a, the cable core 7d acting as a reinforcing memberhas an ellipse-shaped cross-section and encloses one cavity 3, in whichthe fibre ribbons can move freely.

In the case of FIG. 4b, the cable core 7d is rectangular incross-section and encloses three parallel cavities 2d. The whole cablecore with partition walls 10a constitutes in this case a reinforcingmember.

The construction of FIG. 4c corresponds to the construction of FIG. 4bin other respects, except that partition walls 10b between the cavitiesare not reinforcing members there, but the reinforcing member consistsof a peripheral part 10c of the cable core 7d.

In the constructions of the FIGS. 4a to 4c, the cable core 7d (or a partof it, such as the peripheral part 10c of the cable core in FIG. 4c)defines alone the properties of the cable, whereby the surroundingprotective sheath serves only as protection against wear and humidity.Consequently, the protective sheath is not always even necessary.

One advantage of the present invention is also that, by means of asuitable dimensioning of the construction, the difference between themoments of inertia of the cable perpendicular to each other can be madeas great as possible (the cable bends more easily in the direction inwhich the moment of inertia is smaller). The location of the bendingplane of the cable can not always be defined directly by means of thecentre axes of the reinforcing members or by means of other suchfeatures relating expressly to the structure of the cable, but thelocation of the bending plane also depends e.g. on the reinforcingmembers chosen and the dimensions and shapes thereof, on the materialschosen or other such factors defining the intensities of the moments ofinertia.

Though the invention has been described referring to the embodimentsillustrated drawing, it is clear that the invention is not restrictedthereto, but it can be varied in many ways within the scope of theinventive idea described above and in the enclosed claims. For instance,the bending planes of the cable and the fibre ribbon package do not haveto coincide in the manner presented in the examples above, because thelocation of the fibre ribbon package in the cavity and the location ofthe cavity itself in the cable can vary. The same cavity can eveninclude two parallel fibre ribbon packages loosely joined together orthe cable can comprise two or even several parallel cavities, each ofwhich can include one fibre ribbon package or several parallel fibreribbon packages tied together. In principle the cable could also havee.g. two cavities along a diametral axis, each of which includes onefibre ribbon package or several packages. The cavity can also be filledwith fat or pressurized, and to the whole construction can be addedlaminated protective covers or other known protective or reinforcingconstructions.

We claim:
 1. A cable construction comprising a cable having asubstantially circular cross-section with vertical and horizontaldiametral axes, said cable having a cavity extending lengthwise alongthe cable, said cavity being centered on the vertical axis and elongatedin the vertical direction, a package of fibre ribbons in said cavity,each said ribbon including a plurality of parallel optical fibresstacked one on the other in the vertical direction such that the fibresare confined in the horizontal direction but have free play in thevertical direction, and reinforcing means extending lengthwise in saidcable, said reinforcing means being centered on said horizontal axis andbeing substantially symmetrically arranged relative to said verticalaxis to provide less bending resistance around said horizontal axis thanaround said vertical axis, said cable having a core part constitutingsaid reinforcing means, said core part providing said cavitytherewithin.
 2. A cable construction according to claim 1, wherein saidcore part has a substantially ellipse-shaped cross-section.
 3. A cableconstruction according to claim 1, wherein said core part issubstantially rectangular in cross-section and includes a peripheralpart and at least two said cavities in parallel relation, said core partas a whole constituting the reinforcing means.
 4. A cable constructionaccording to claim 1, wherein said core part is substantiallyrectangular in cross-section and includes a peripheral part andpartition walls enclosing at least two said cavities in parallelrelation, said peripheral part constituting the reinforcing means.
 5. Acable construction according to claim 4, wherein said cable has an outersurface provided with grooved, reinforcement portions arranged solely inareas closet to ends of the cavity on said vertical axis.
 6. A cableconstruction comprising a cable having a substantially circularcross-section with vertical and horizontal diametral axes, said cablehaving a cavity extending lengthwise along the cable, said cavity beingcentered on the vertical axis and elongated in the vertical direction, apackage of fibre ribbons in said cavity, each said ribbon including aplurality of parallel optical fibres stacked one on the other in thevertical direction such that the fibres are confined in the horizontaldirection but have free play in the vertical direction, and reinforcingmeans extending lengthwise in said cable, said reinforcing means beingcentered on said horizontal axis and being substantially symmetricallyarranged relative to said vertical axis to provide less bendingresistance around said horizontal axis than around said vertical axis,said reinforcing means comprising a profile member centrally arranged insaid cable and providing said cavity therewithin.
 7. A cableconstruction according to claim 6, wherein said cable has an outersurface provided with grooved, reinforcement portions arranged solely inareas closest to ends of the cavity on said vertical axis.